Convenient and Universal Fabrication Method for Antibody–Enzyme Complexes as Sensing Elements Using the SpyCatcher/SpyTag System

Antibody–enzyme complexes (AECs) are ideal sensing elements, especially when oxidoreductases are used as the enzymes in the complex, with the potential to carry out rapid electrochemical measurements. However, conventional methods for the fabrication of AECs, including direct fusion and chemical conjugation, are associated with issues regarding the generation of insoluble aggregates and production of homogeneous AECs. Here, we developed a convenient and universal method for the fabrication of homogeneous AECs using the SpyCatcher/SpyTag system. We used an anti-epidermal growth factor receptor (EGFR) variable domain of a heavy chain antibody (VHH) and a glucose dehydrogenase (GDH) derived from Aspergillus flavus (AfGDH) as the model antibody and enzyme, respectively. Both SpyTag-fused VHH and SpyCatcher-fused AfGDH were successfully prepared using an Escherichia coli expression system, whereas anti-EGFR AECs were produced by simply mixing the two fusion proteins. A bivalent AEC, AfGDH with two VHH at both terminals, was also prepared and exhibited an increased affinity. A soluble EGFR was successfully detected in a dose-dependent manner using immobilized anti-EGFR immunoglobulin G (IgG) and bivalent AEC. We also confirmed the universality of this AEC fabricating method by applying it to another VHH. This method results in the convenient and universal preparation of sensing elements with the potential for electrochemical measurement.